There are a variety of specialized subsets of human dendritic cells (DCs) and monocytes, but full understanding of their diversity and ontogenic relationships has been hampered by the technologies used to assess them. In Science, Villani et al. use single-cell RNA sequencing combined with dimensionality reduction and functional analysis to produce a comprehensive mRNA-expression 'atlas' of DCs and monocytes from human peripheral blood. Through analysis of approximately 2,400 individual cells, six distinct subsets of DCs emerge, five of which correspond to established populations, with a sixth previously unknown population characterized by expression of the receptor tyrosine kinase AXL and particular SIGLEC receptors. That sixth subset is called 'AS-DC' and has a continuum of features including, to varying extents, those of plasmacytoid DCs (pDCs) and conventional DCs. For example, some AS-DCs are able to efficiently stimulate T cells but also express pDC markers, which suggests that they might be a confounding contaminant of procedures used to sort conventional pDCs.

Science (21 April 2017) doi:10.1126/science.aah4573